Air gases have a great industrial interest because they have many applications in very different technical fields: production of steel, glass or paper, medicine, metal welding, combustion or pollution control, for example.
One of the techniques used at present to produce these gases is the so-called "PSA" (Pressure Swing Adsorption) technique, which covers not only PSA processes as such, but also similar processes, such as VSA processes (Vacuum Swing Adsorption) or MPSA (Mixed Pressure Swing Adsorption). According to this PSA technique, when the gaseous mixture to be separated is air and the component to be recovered is oxygen, said oxygen is separated from said gaseous mixture by preferential adsorption of at least the nitrogen on a material that is preferentially adsorbent to nitrogen, said adsorption being carried out by pressure variation applied in the separation zone containing said adsorbent material. The oxygen that is not at all or little adsorbed is recovered at the outlet of said separation zone; the latter has a purity in general greater than 90%, even 93% or more.
More generally, a PSA process for the non-cryogenic separation of a gaseous mixture comprising a first compound that adsorbs preferentially to an adsorbent material and a second compound that adsorbs less preferentially to said adsorbent material than the first compound, for the production of said second compound, comprises in a cyclic manner:
a step of preferential adsorption of at least one said first compound on said adsorbent material, at an adsorption pressure called "high pressure", with recovery of at least a portion of the second compound thus produced; PA1 a step of desorption of the first compound thus trapped by the adsorbent, at a desorption pressure lower than the adsorption pressure, called "low pressure"; PA1 a step of repressurization of the separation zone comprising the adsorbent, by progressive passage from said low pressure to said high pressure. PA1 the ratio (.sigma..sub.c /.mu..sub.c) is comprised between 0.02 and 0.10, preferably between about 0.03 and 0.08 PA1 the ratio (.sigma..sub.s /.mu..sub.s) is comprised between about 0.02 and 0.20, preferably between about 0.03 and 0.18 PA1 it is selected from the group consisting of exchanged zeolites and non-exchanged zeolites. PA1 it is selected from the group consisting of zeolites of types X, Y, A, ZSM-3, ZSM-5, mordenite, faujasite or clinoptilolite. PA1 it contains cations selected from the group consisting of cations of lithium, calcium, zinc, copper, manganese, magnesium, nickel, potassium, strontium or any alkaline metal or alkaline-earth metal, and their mixtures. PA1 it contains at least 50% of cations of lithium and/or at least 10% of calcium cations and/or at least 5% of zinc cations. PA1 it has a ratio Si/Al of 1 to 1.25 and preferably about 1, and is preferably a zeolite LSX. PA1 the gas flow to be separated comprises oxygen and nitrogen, preferably the gas flow is air; the air, in the framework of the present invention, being contained within a building or an enclosure that is heated or not, or outside air, which is to say under atmospheric conditions, taken as is or if desired pretreated, PA1 the first component is nitrogen and the second component is oxygen; and a gas flow rich in oxygen is produced, which is to say comprising generally at least 90% of oxygen. PA1 it is of the VSA type, PA1 the high adsorption pressure is comprised between 10.sup.5 Pa and 10.sup.7 Pa, preferably of the order of 10.sup.5 Pa to 10.sup.6 Pa, and/or the low pressure of desorption is comprised between 10.sup.4 Pa and 10.sup.6 Pa, preferably of the order of 10.sup.4 Pa to 10.sup.5 Pa. PA1 the supply temperature is comprised between 10.degree. C. and 80.degree. C., preferably between 25.degree. C. and 60.degree. C. PA1 a cycle with two adsorbers (A and B) of about 2.times.40s with high pressure of 1.4 bars and low pressure of 0.4 bar; PA1 a cycle with three adsorbers (A, B and C) of about 3.times.30s with high pressure of 1.1 bars and low pressure of 0.3 bar.
However, it is known that the efficiency of separation of a gaseous mixture, such as air, depends on numerous parameters, particularly the high pressure, the low pressure, the type of adsorbent material used and the affinity of the latter for the compounds to be separated, the composition of the gaseous mixture to be separated, the temperature of adsorption of the mixture to be separated, the size and shape of the particles of adsorbent, the composition of the particles and the temperature gradient within said adsorbent bed.
Until now, although no general law of behavior has been determined, given that it is very difficult to interconnect the different parameters, it is also known that the nature and the properties of the adsorbent play an important role in the overall efficiency of processes of the PSA type.
At present, the zeolites, particularly types A, X, Y or LSX (Low Silica X), are the most common adsorbents used in PSA processes. The zeolite particles contain usually mono, di and/or trivalent cations, for example cations of alkali metals, alkaline-earth metals, or lanthanides, incorporated during the synthesis of the particles of zeolite and/or subsequently added by an ionic exchange technique.
Ion exchange is generally carried out by placing the non-exchanged or raw zeolite particles into contact with a solution of one or several metallic salts comprising the cation or cations to be incorporated in the zeolitic structure and subsequent recovery of the exchanged zeolite, which is to say zeolite containing a given quantity of metallic cations. The proportion of metallic cations introduced into the zeolitic structure is called the exchange load.
Conventionally, it is recommended to try to obtain a perfectly homogeneous adsorbent, which is to say having no or the least possible variations not only of exchange load, but also of capacity and/or selectivity; the capacity and the selectivity of the adsorbent being parameters known to those in the art and defined in numerous publications, particularly U.S. Pat. No. 4,481,018, EP-A-0 589 406 or EP-A-0 598 391.
The concept of homogeneous adsorbent is clear from EP-A-0 589 406 and EP-A-0 598 391, given that these latter teach the use in a PSA process of an adsorbent having a given capacity and selectivity and chosen within a certain range of values. In other words, these publications neither teach nor take account of the existence of possible fluctuations of capacity and/or selectivity of the adsorbent, which is to say a heterogeneity of these latter as to one and/or the other of these two parameters.
As a result, at present, the adsorbents said to be good adsorbents for the separation of gases, particularly for the separation of air gases by a PSA type process, are those whose capacity and/or selectivity are the most homogeneous possible.
More generally, no prior art publication has shown or emphasized, until now, the importance that can attach to the degree of heterogeneity of capacity and/or selectivity of a given adsorbent, and the impact of this degree of heterogeneity particularly on the performance of a PSA process using such a heterogeneous adsorbent.
Moreover, the adsorbents most used in processes for the separation of gases, in particular air, are strongly exchanged zeolites, generally more than 80%, with very costly metal cations, such as particularly lithium cations. In this regard can be mentioned U.S. Pat. Nos. 5,268,023 and 5,152,813.
It will thus be immediately apparent that the fact of not being able to use other than zeolites for the separation of gaseous mixtures, particularly strongly exchanged ones, having a homogeneous capacity for adsorption and selectivity of adsorption, carries with it the requirement for rigorous quality control and meticulous selection of the adsorbents after their production. There is thus a high rejection rate and an inevitable and considerable increase in the overall cost of production and cost of the process of separation of the gases thus produced.